Creep in Glassy Polymers

Creep in plastics is one manifestation of their viscoelastic nature, which is, in turn, a characteristic feature of the macroscopic deformation behaviour of polymeric materials in general. The term ‘creep’ was adapted from its use in relation to the behaviour of metals, for which it designates the time-dependent component of the strain that is observed as a result of prolonged stressing. The initial, nearly instantaneous, strain is elastic in nature and that which develops later is very largely ‘plastic’, or irreversible. No such convenient and logical subdivision of the deformation of plastics is possible. Most aspects of their deformational behaviour are so severely time-dependent that the isolation of a distinct elastic component or a region of genuinely constant modulus is possible only under very special experimental conditions and ‘creep’ means the entire strain caused by an applied force. In contrast to its use in relation to metals, there is no implication that the deformations are irreversible, despite the broad classification of ‘viscoelastic’ in which the conjunction of ‘visco’ and ‘elastic’ could be taken to imply a viscous, or irreversible constituent. It is apparent that words and definitions are often chosen for their pictorial quality rather than for their explicit meaning, even in scientific contexts.